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Topic Title: Tail size
Topic Summary: What size tails allowed for auxiliary DB
Created On: 18 April 2013 02:06 PM
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 18 April 2013 02:06 PM
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Thripster

Posts: 648
Joined: 22 August 2006

Hoping somebody can help me please.

Domestic property with 60A fuse at cut out. From here 16mm² tails to meter, then 25mm² tails from meter to existing DB. House owner wants power to garage (20A sockets and 6A lights). No capacity in existing DB so tails have been split, Henley's used to allow feed to fault current DB at meter cupboard. Space is tight, On Site Guide says 25mm² minimum but I would like to use 16mm² to feed this circuit - where does it say in the regs that 25mm² has to be used, irrespective of fuse capacity at cut out/ I am sure there is a table/words but cannot find it.

Thanks and regards
 18 April 2013 02:18 PM
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geoffsd

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It doesn't. The OSG is wrong.

Table 4D1A in BGB.
 18 April 2013 02:22 PM
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AJJewsbury

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Tails are sized in the same way as for any other circuit - i.e. they need to have adequate current carrying capacity for the load and (usually) co-ordinated with the protective device(s) to give overload and fault protection. OSG is intended for 100A loads, not 26A.

Given that 16mm2 earth is adequate with 25mm tails, 16mm L+N tails will be protected from faults by the supplier's fuse by the same logic. 16mm2 will have no problem carrying 26A and overload protection is provided by the downstream CU.

I don't see a problem with 16mm2 tails.

- Andy.
 18 April 2013 02:28 PM
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Thripster

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Thanks Geoff - that is all I needed to know. I assume that the OSG makes the stipulation of 25mm² tails on the basis of a 100A fuse. Do we need to let the compilers of the OSG know or is it so obvious that i should have picked up on it in the first place?

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 18 April 2013 02:32 PM
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perspicacious

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Would the DNO/meter chaps be happy seeing say 500 mm of 25 mm2 "meter tails" to a Henley block with say 4 pairs of 10 mm2 "meter tails" to adjacent CUs with 50 A of installed circuit-breakers in each?

Do they have anything written in their requirements for "meter tail" size v cut-out fuse size?

How is 537.1.4 being interpreted these days?

Regards

BOD
 18 April 2013 02:34 PM
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Thripster

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Thank you Andy. Being on site without the BGB - I know that 16mm² is good for the load/fault current considerations but you do a double take when looking at the OSG. Perhaps the OSG should make it clear that these comments relate to a 100A head.

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 18 April 2013 02:40 PM
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Thripster

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Yes, interesting BOD. I have been asking customers to contact their supply company and get them to uprate tails to 25mm² (where necessary) and fit a double pole main switch. This is done FOC upon occasion (Northants) but on other occasions there is a charge (pecuniary)..........

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 18 April 2013 02:46 PM
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geoffsd

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Originally posted by: Thripster I assume that the OSG makes the stipulation of 25mm² tails on the basis of a 100A fuse.

I think we assume so, but it doesn't say that.

Do we need to let the compilers of the OSG know

We probably think someone else must have but it was the same in the last edition.
So ...
 18 April 2013 02:59 PM
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Thripster

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Makes you think, in this and other cases, I wonder if this domestic customer will wear the £100 parts it will cost if I have to fit a 'Main Switch' (if the DNO will not do it ) as a result of this work? I am sure the domestic customer will jump at the chance knowing that their installation will comply with the regulations. Or, shall we assume that two main switches mounted in domestic consumer units fixed side by side are adequate for isolation in a domestic environment? That is, should there be a fire and the firemen turn up, will they switch off one DB and ignore the other, mounted next to it, as they only expect to see one Main Switch?


Regards
 18 April 2013 03:02 PM
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ebee

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I know YEDL always put a 80A fuse in their cut out (even if there was a 100A one in) because they give their contractors 16.0 tails for their bit.
They don`t mind if we fit 25.0 tails or 16.0.
Tails length limit I think is 3metres.

In that case if you have 16.0 tails or a combination of 16.0 and 25.0 and a tail block then a 80A or 60A fuse should be good for tails not exceeding 3m radially to each CU or switchfuse

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 18 April 2013 03:15 PM
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perspicacious

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"as they only expect to see one Main Switch?"

What is the first word of 537.1.4?

Regards

BOD
 18 April 2013 03:31 PM
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ebee

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"A" as opposed to "The" perhaps?
"A" being one of any rather than "the" being The only?

Some day you can have a right good game of "Let`s guess what they meant when they wrote this reg"

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 18 April 2013 03:35 PM
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dg66

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If the tails to the garage CU are more than 3mtrs long you need to provide a protective device.

-------------------------
Regards

Dave(not Cockburn)
 18 April 2013 03:37 PM
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Thripster

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Yes Bod, you cannot refute the wording and I wouldn't dare to. However, you can argue about whether it is necessary and whether (for the sake of £100 plus labour) it might be better in these circumstances to note it as an exception on the certificate if the owner will not wear the cost. It contravenes BS regs but that does not make the installation unsafe. As I say, I always get the customer to get their supply company to fit a master switch.

Regards
 18 April 2013 03:41 PM
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Parsley

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GN5's take on this below

A main switch is required for each source of supply to an installation. These main switches may be arranged such that they can be operated at the same time, via a suitable interlock arrangement, to provide disconnection of the installation from all sources of supply. Where this is not the case, a warning notice will be required informing of the need to operate all main switches to fully isolate the installation.
This situation frequently occurs when an additional consumer unit is added, e.g. to supply an electric shower.


So how many installations can be fed from one source?
 18 April 2013 03:41 PM
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AJJewsbury

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It contravenes BS regs

There is a school of thought that says it doesn't - they just say that the building contains two electrical installations, each fed by its own CU, each with its own single main switch (in each of the CUs).

- Andy.
 18 April 2013 04:01 PM
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perspicacious

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"However, you can argue about whether it is necessary and whether (for the sake of £100 plus labour) it might be better in these circumstances to note it as an exception on the certificate if the owner will not wear the cost. It contravenes BS regs but that does not make the installation unsafe."

A rather bold statement about safety if read in conjunction with the paragraph at the top of p12 and noting an "exception" still puts the onus on the signatory for compliance with the last three lines of 120.3.

Regards

BOD
 18 April 2013 04:07 PM
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perspicacious

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"So how many installations can be fed from one source?"

"There is a school of thought that says it doesn't - they just say that the building contains two electrical installations, each fed by its own CU, each with its own single main switch (in each of the CUs)."

The days of 6 bits of 2.5 or 4 mm2 equivalent twisted together and put into the bottom of the meter to feed half a dozen switchfuses with one or two OPD are back

Regards

BOD
 18 April 2013 04:45 PM
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AJJewsbury

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The days of 6 bits of 2.5 or 4 mm2 equivalent twisted together and put into the bottom of the meter to feed half a dozen switchfuses with one or two OPD are back

I'm not sure they ever fully went away.

Besides it made things much more interesting! My grandmother's house was just like that - as a young lad I remember looking up at it all trying to figure out how it all worked. Kids these days with just a single impenetrable box to look at are loosing out I'm sure!!

- Andy.
 18 April 2013 05:02 PM
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Parsley

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Originally posted by: AJJewsbury

The days of 6 bits of 2.5 or 4 mm2 equivalent twisted together and put into the bottom of the meter to feed half a dozen switchfuses with one or two OPD are back


I'm not sure they ever fully went away.



Besides it made things much more interesting! My grandmother's house was just like that - as a young lad I remember looking up at it all trying to figure out how it all worked. Kids these days with just a single impenetrable box to look at are loosing out I'm sure!!



- Andy.


My parents house was like that, tails and henley blocks everywhere. My dad was at the forefront of energy conservation way before part L. He had a separate meter on the immersion heater and used to keep a house log book which he meticulously updated every week.

Regards
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